Creeping thymes or wooly thyme are my fave groundcovers. They seem to thrive on abuse. Some varieties spread pretty quickly, so you don't need to buy a lot of plants. If you know someone who has some, you can probably get some cuttings for free--they are easy to start from cuttings.

You might also want to ask over at the groundcovers forum. You'll likely get several other interesting ideas over there:

Here is a link that might be useful: Gardenweb Groundcovers Forum

Thank you!

Aggressive ground covers I have under cedar trees-
Geranium macrorrhizum- drought tolerant, pink flowers
Symphytum iberis- drought tolerant, tubular blue to pink to white flowers in spring

not aggressive but very nice if you have the time to weed and water to get it established-
Gaultheria procumbens- wintergreen- choice shiny evergreen leaves, slow spreading, tubular flowers and red edible berries.

You might want to look at the Stepables.com website for photos and info on ground covers.

Rubus pentalobus - called Taiwan creeper, creeping berry, etc. Planted it beneath huge sycamore tree 3 yrs ago and it has filled in, right up to trunk. Loves that dry shade. Takes abuse - rake leaves off it every Fall.