If we only needed one dress for a wedding, life would be less fun. In fact, we need several. There's the bridal shower, which for most, requires a light hearted little number, not to formal, the equivalent of a sun dress with a little panache.
Then there's the rehearsal dinner dress. For most, this dress will be more formal. You can't wear a sundress after dark, so you need something with a little more slink to it. Not too sexy, but clearly evening. I'd say the rehearsal dinner usually qualifies as "semi-formal", at least for the bride and groom.
I loved my rehearsal dinner dress almost as much as my wedding gown. I got it at Anthropologie, so I thought that would be a good place to start. I found several shower and dinner worthy dresses there. I also found several options at BCBG. The dresses there are so unique season after season. I recommend you check out both. I love the retro stripe number from Anthropologie for a shower, and my favorite for a rehearsal dinner dress is that royal blue stunner at the bottom right. I can't think of anyone who wouldn't look great in that color.