Rei's MinimapGives you a minimap and lets you set waypoints. Has some other features too, like highlighting slime chunks.
SomniaWhen you sleep, time still passes at an accelerated rate. No more spawning creatures nearby because you're not inside! Sleeping while machines are working will let them get a lot of work done while you nap.
BuildcraftJust too much to list here. Pipes, oil, engines, etc. Very nice mod, lets you transport items and set up large machines.
IndustrialCraft2Adds electrical power, machine blocks, material processing and many other things.
Equivalent ExchangeA very well done alchemy mod which allows you to transmute materials as well as work towards some truly endgame items.
Redpower/RedworldA fantastic set of mods which changes world generation to allow volcanos, adds in a few ores and things like basalt and marble, and adds in a new wiring system as well as bluetricity machines. (Note: By default I have set redworld not to spawn copper or tin ore because IC2 will spawn them. If you allow it to spawn these ores you will have two 'different' copper and tin ore/ingot types. While these types are interchangable due to the forge dictionary, they are still annoying to deal with. If you want them to be spawned open the redpower.cfg file in the redpower folder within .minecraft and change "copper=0" to "copper=1" and "tin=0" to "tin=1"
ThaumcraftAdds some fantastic options for breaking down items into raw magic and using it to set up runes and symbols with various effects. Also custom crafted travel portals!
Energy CouplersAdds in a few couplers which can transform Buildcraft energy to IC2 electricity and back. Very useful.
TreecapitatorTrees fall when their bottom block is broken by an axe!
Minefactory ReloadedI haven't messed with this much yet but it looks fantastic. For more info see the linked page.
Not Enough ItemsA very nice replacement for TMI which also doubles as a recipe book. Check out the linked page for details.
Single Player CommandsAdds SMP type commands to singleplayer.
Minecraft ConsoleAdds in a console! No longer do you have to use the silly chat interface for commands. (Currently accessible by hitting \ )
ForestryAdds in machines which are compatible with Buildcraft and will automatically grow and harvest trees, crops and even mushrooms I believe.
MCForgeRequirement for most of these mods.
ModLoaderRequirement for most of these mods. Allows us to load mods from the mods folder!
ModLoader MPRequirement for most of these mods.