Your voice, your touch, your support and knowledge. Those are the MOST important things you can bring. I have massage oil and rice socks, and tennis balls, too...but my words of encouragement and my knowledge of comfort measures and ways to turn the techincal into english are most often offered up as the most valuable things that I "had with me" during labor and birth when we "decompress" during post partum visits. Really.
The MOST often appreciated thing that women I've attended appreciate it my voice softly in their ear telling them that they're doing great, that they ARE doing this, that we'll take it one contraction at a time, that their body is amazing, reminding them to relax (they've all liked progressive relaxation, almost every single one) and on and on. They tell me over and over again that, if there was any noise or confusion at all, they really heard my quiet voice in their ear, and blocked the rest of it out. I think that that speaks VOLUMES to what we're really there for...simply to support the mama, and most often, I've found the emotional support to be the most necessary.
If you help to provide an environment where they are more able to move freely and choose their own "comfortable" positions, often they'll do a great job of THAT themselves (notice I say often, not ALWAYS). But there is seldom anybody else there just quietly telling them that they're wonderful. And that's what I find they need/like/want the most, through feedback from my mamas.
Hope that wasn't too rambling.