Treeless saddles have been around forever, got overtaken by treed saddles, but are making a comeback!
Which one you get depends on your horse's conformation, your preference, and what type of riding you are likely to be doing with it.
Not all horses, nor all riders, will like them and some may favour certain styles over others. The only way to know for sure is to try as many different types as you can.
Treeless saddles do require a certain amount of fitting and the fit needs checking frequently, as the horse will gain and loose weight and muscle throughout the year.
Incorrectly fitted saddles, whether it be treed or treeless, can cause damage to your horse if not fitted correctly or used in a way not recommended by the manufacturers.
Representatives or agents of Treeless saddles can assess and advise you on padding and suitability of your horse for their particular saddle.
Don't just settle for the first one you try or the cheapest, listen to your horse, they will let you know which one they prefer.
There are a lot of copies of named brands on the market available incredibly cheaply - DO NOT be fooled into thinking that because they look like a popular and well-thought-of saddle they will be the same. Chances are they've been very cheaply produced abroad with substandard materials, quality and attention to detail and won't have been tested to ensure their safety and pressure dispersing qualities.
The various types on the market at the moment (that I'm aware of - please email me if you know of more!) are:
The Treefree saddle is made in England. It has a gullet and interchangeable pommel and cantle blocks, which come in different sizes to suit your horse. They come in 3 styles, The dartmoor, Exmoor, which has knee rolls and the Western.
Dartmoor Treefree

Exmoor Treefree

The Torsion is very similar to the Treefree, and it comes in many more styles. The saddles can be seen on the Torsion UK website.
Torsion duo saddle with HAF pad and Barefoot Sheepskin seat
The Barefoot saddle comes in differing styles and most have interchangeable pommel and cantle blocks available in differing sizes to fit your horse. The newer style barefoot saddles come with the VPS System (Vertebrae Protection System).
Barefoot Arizona

There are now 4 models of Trekker treeless available. The Talent, the Endurance, the Dressage Master and brand new out, the Bascule.
The Copperbeech treeless is currently very similar in style to the Trekker however new changes are being made after consultation with equine therapists and physiotherapists.
The Startrekk saddles come in many styles, English,Western, Endurance and Iberian/Spanish. The saddles have changeable pommel arches that come in 5 different sizes, 2 choices of stirrup positioning and velcro panels custimise fit on your horse. The saddles have a gullet and panels and are more Traditional in apperance.
Startrekk Western

Startrekk Dressage

The Freeform is more traditional in appearance than most treeless saddles it has interchangeable seats and stirrup positions. It comes in a Classic or Short Back version and there is a variety of models available with extra options such as larger knee rolls,poleys and adjustable girthing.
Freeform SB with forward girthing

The Heather Moffett Flexion SBS saddle is no longer available and has been replaced by a part treed saddle called the Fhoenix. SBS saddles can still be purchased secondhand.
The Cashel Soft Saddle is the smallest and lightest. No adjustable parts - in this manner, it is similar to a Bareback Pad.
The Bob Marshall is an American saddle. They are Western-styled and look very "normal".
The Ansur is made in America. They come in different styles including English and Western and are a Traditional looking saddle. Some models include gullets/panels.
Ansur Konklusion

Ansur Classic

The Solution Saddle is the English made verison of the Ansur saddle. They have different style of english saddles and some models have a gullet/panels.
The Awareness is another "normal" looking saddle. This saddle is available in New Zealand.
The Diddi Pad is a traditional looking saddle designed for Icelandic horses. It has panels and a gullet.

Another Icelandic treeless is the Galsi.
The Rebecca Treeless, or Softride comes in 2 styles and is distinctly Western in appearance.
The Baretek saddle looks very traditional when on the horse, and comes in English or Western appearance. It is now available in the UK.
The Kay Humphries Mondial Flexion is a new treeless saddle on the market. It's the most expensive, and looks exactly like a treed saddle. It comes in Dressage, Jumping and Working Hunter.
Hilason treeless saddles are widely available on Ebay. They do copies of most Western and English styled treeless saddles.
The Sensation treeless saddle is a traditonal styled saddle which comes in English or Western style with different models. There are the G3 or G4 models, which have differnt girthing options.
It has a gullet, changeable girthing (G3 or G4) and different stirrup positions which can be changed to suit the riders individual needs.
The endurance stirrup position attaches to the bottom of the saddle flap and gives no pressure points on the horses back.
The Hard Use stirrup postion attaches under the seat on a velcro plate and to the bottom of the saddle flap, this distrubutes the weight between the seat and the saddle flap.
The Freeswing stirrup position attaches to the velcro plate under the seat and gives a traditional Engliosh stirrup position. The velcro plate can be moved to suit the rider's preference.
Sensation English Trail with G3 girthing and Hard Use stirrup position

Sensation English Trail with G3 girthing and Hard Use stirrup position

Sensation Hybrid saddle with G3 girthing and endurance stirrup position
Bareback Treeless Saddles from South West Saddlery.
The Keilir is another Icelandic treeless saddle but most unlike the Diddi Pad or Galsi in appearance.
The Boz Bareback Saddle looks like a bareback pad but has received very impressive write ups as a saddle.
The Ghost Saddle is new on the market and receiving very good reviews.
The Libra Trail saddle is a good introductory saddle. It comes in nubuck or synthetic models and has a removable pommel and cantle block.
A traditional looking treeless dressage saddle is the Equinelibrium Supreme 2. Equinelibrium have also released a general purpose model treeless saddle.