I haven't seen any others, but
Boom has worked well for me.
Another vote for Boom, with a caveat.
I don't know if anyone else is having this problem, but while Boom does a great job amplifying the MBP speakers, it has totally screwed up my Sound Effects. For example, on Mac Mail, I don't get my incoming alert or the Send mail "whoosh". Other sound effects are wonky, too.
An interesting phenomenon; when I am using Face Time, the Mail sound effects work fine. Boom is still activated. If I quit boom, and I turn up the volume on internal speakers, I can also hear sound effects.
I've learned to live without the sound effects, but I do miss the incoming mail alert. As I said, I don't know if this is unique to me. I have contacted the devs on a couple of occasions, but nothing has been done, which leads me to believe it is not a widespread problem. I have tried everything, short of a clean re-install of Lion. (I'm running 10.7.3)
Try Boom, you probably won't have my problem.