What size HD do you have? AFAIK, around 15% free space is needed on a HD. Do you know roughly where all the space has gone? By that I mean do you store a large amount of photos/mp3/movie files?
Firstly you're going to have to free at least 15% up and you might need to purchase an external HD which isn't such a bad idea as nowadays they have come right down in price and come in very handy.
I recommend you download and run a maintenance app like OnyX, and have it clean out all your caches and the clean up might free up *some* space ... here's the link for O
nyX Ver 1.85. The Tiger version is the 3rd from the top.
You haven't said which model eMac you have so I can't recommend much but I at least know that depending on which model you have, you'll have from between 40GB to 160GB as that's the range of capacity unless the original HD has been upgraded, which brings me to my point .... you could consider putting a larger capacity HD in ... I haven't checked but it might be do-able.
I know Mac OS X has a Language Packs which take up some space and there's apps like GarageBand, iMovie, iDVD etc that you could uninstall.
If you have the original install discs you can always reinstall them later on if need be.