Empire of the Petal Throne (EPT) published by TSR is a D&D-derived system with hit dice and levels using d100. It's a simple system, and it takes about 10 minutes to create a character. This game contains a very good amount of the setting details in a pretty small number of pages.
Swords and Glory (S&G) published by Gamescience comes in two parts. The Tekumel Sourcebook is a setting book with no game rules. It reads like an anthropological text, but it is the most complete description of Tekumel in existence. The Player's Handbook contains all the rules needed to play. It's a very complex system with both random die rolls and point-buy skills and spells that has a modifier for everything. A Referee's Handbook was planned, but never produced.
Gardasiyal published by TOME is simplified version of the Swords and Glory system. It relies on the Adventures on Tekumel series of solitaire adventures for character generation and background. A comprehensive Tekumel Bestiary was also published during this time.
Tekumel: Empire of the Petal Throne (T:EPT) published by Guardians of Order is derived from the Tri-Stat dX system of the same company. It is a rules-heavy point-buy system. One of the drawbacks is that the bestiary is limited, only a few non-human races are detailed, and the spell list is incomplete. Future volumes that were never produced would have filled in the gaps.
There is one more official ruleset, Bethorm by Jeff Dee, published last fall after a Kickstarter campaign. It is a rules-light point-buy system about on the same range of complexity as GURPS Lite or BRP. Two of its features are drawings of nearly every creature, drawn by Jeff Dee, as well as translations of all of the spells from the game with the most complete spell corpus, Swords and Glory.
The five systems in order of complexity (lowest to highest): EPT, Bethorm, Gardasiyal, T:EPT, S&G.