Foxy

Depends on brand. The best tip I can give you is to select a car first, then download the manual from the website of the manufacturer, in the first few pages it will tell you exactly what you need. Here is a short list of stuff it's good to have...

Needle nose pliers
Heavy duty pliers
Accurate calipers
Good set of Hex wrenches
Good set of screwdrivers
Threadlock (blue NOT RED!!)
CA (Superglue)
Grease
Machine oil (bearing oil)
Side cutters
Large (stanley type) knife
Small scalpel type knife
Heavy duty scissors
Curved body scissors
Reamer
Tweezers
Dremel
Sandpaper
Flywheel tool (ask your LHS)
Sticky pads (like the ones they stick posters up with)
Sticky velcro

This may sound like a lot, but most of it is stuff you should have around anyway if you have an interest in building stuff. Good kits use hex hardware (with very few exceptions), so the good set of hex wrenches is absolutely the MOST important thing and worth spending money on. HUDY make the best RC tools, if you can get a set of their hex wrenches you will be well covered. If you take one of the cheaper kits, it will probably be mostly philips screws and the hex wrenches will be less important (only the engines ALWAYS use hex). Allen keys will NOT do. I can't stress enough the importance of good hex tools.

[X(] Nearly forgot the glowplug wrench! Also known as a cross wrench. An adjustable spanner (small) is useful too. And a 7mm nut driver if it's not on the cross wrench (it usually is).