Popular drivers
For a complete list of drivers, definitely check out the flashlight driver database and this CPF thread. But here are a few drivers from budget dealers that people have found useful when modding or upgrading flashlights. All of these drivers are 17mm (good for P60 drop-ins) and use a micro-controller and PWM for lower modes.
NANJG 112A 0.9-4.2V 1000mA boost driver from KD with stars on the back that can be soldered to provide different modes. Supposedly buck/boost with an inductor. Seems well-regulated since current draw increases as voltage of li-ion decreases, but current draw declines with voltage on 1.2V NiMH. Works with AA or li-ion, but with 2xAA or 1xCR123A the low-voltage warning will cause flashing. 3-mode grouping has low and medium pretty close together. This driver doesn't seem to be available anymore though. Review KD
NANJG AK-47 3x7135 2.6-4.5V 1000mA linear regulator. Two versions, one by DX (5-mode) and one by KD (2-mode) with unadvertised mode groups available. Both have stars on back that can be soldered for different groups like the 112A. Modes are nicely spaced at 5, 30, and 100%. Efficiency is very high, 85-90% on High. Newer DX versions seem to have dropped the ATMEL chip for a PIC.[1] DX KD CPF Post Review
NANJG ROHS 101-AK 2.8-6V linear regulator. Two versions, one by DX is a 3x7135 1050mAh driver with 3 (LMH), 5, or 7 modes selectable by clicky. Has room on board to add 4th 7135 chip for 1400mAh output DX, but on at least one recent order the buyer received a NANJG AK47 instead. The other version of this driver is a 3-mode (LMH) version with 4x7135 on it already from Shining Beam with modes of 8%, 25%, and 100% with maximum output of 1400mAh. Review
NANJG 106 8x7135 linear regulator driver advertised at KD as 1100mA, but should be 2800mA. Runs off a single lithium-ion battery (which won't stay at 2.8A for long). Advertised as 5 modes, but has stars on the back similar to the other NANJG drivers that allow different groups of modes to be hardwired. KD Shiningbeam
Programmable Drivers
Some drivers, like the 112A, AK-47 and the 101-AK above have an Atmel Tiny13 chip on board. It is possible to reprogram this chip using AVR equipment and software to get not only exactly the number of modes and the light levels you want, but also to program the driver so the light has a programmable UI. More information here. Hope to have a page here on the Wiki to get people started.