If you're playing B/X or something similar, but perhaps you're a bit of a "roll player" (like me!), here's a quick little bolt-on system to bring some extra action into your combats. It is built on the basic principles of the DCC Mighty Deeds™ combat stunt system.
Step 1: Bump the thief up to a d6 hit die. Let's face it, he could use the boost.
Step 2: Use this for attack rolls:
1d16 + HD, i.e.
For stunts, you have to declare them before rolling, get a 4+ on the hit die and a high enough total roll to score a hit. If you do not meet both conditions it's a miss. If you do meet both conditions, you can choose between scoring double damage (called shot), or scoring normal damage plus applying an effect, such as disarming, knocking down, pushing back, etc.
There's no advancement built in, so if you want the PCs to improve, you'll have to figure that out yourself. I figure a magic sword or whatever would allow you to replace the d16 with a d20 (more or less equivalent to a +2). I figure this is enough of an improvement over the regular system that we can just ditch the improvement over time. If you want to see the mathematical effect, check this graph.
For crits, you can do whatever, but I'd probably just do natural 16's for the sake of simplicity and giving a very minor bump to crit chances. Maybe even have fighters crit on 15-16 so they can be real murder machines.
If for some weird reason you don't already have several sets of zocchi dice, d16s can be bought individually.