Hero System/Champions and Gurps are two other systems which are similar to each other, and very different in terms of style and mechanics to D&D. Both are point-based character systems, using 3d6 as their main task resolution mechanic. Both allow very detailed characters, and much differentiation between characters. Both are also classless systems.
Rolemaster is another detailed system, with significant differences to D&D. Die rolls are open-ended (you roll percentiles, if you roll 96+, you roll again and add to the original roll). Combat uses tables, similar to AD&D1, but instead of looking up the target number to hit, you roll, cross-reference with armour type on the right weapon table, and find the damage you deal. Crits have their own tables. Skills are quite granular, compared to D&D4 or even D&D3/.5.