Boosting MP3 volume

[Father Ted]

OC gurus - I need your help.

As my MP3 collection has grown the difference in volume bewteen some of the tracks has become more apparent. Some seem to be really quiet which means constantly adjusting the volume up and down as I play them.
Can anyone recommend some free software that would enable me to boost the volume of the quiet tracks wothout adversely affecting their quality.
Any help would be greatly appreciated, cheers.

 

[Father Ted]

Thanks for all the advice folks, and forgive my ignorance, but using replay gain will add tags, will my portable mp3 player be able to read these tags? (I hope that's not a ridiculous question)

 

[Father Ted]

Thanks for all the help guys, I'm going to give MPGain a whirl