Does anyone know how to...

[Superior Beatslayer]

Download ringtunes from something like Limewire and put on your cellphone? $3.50 for a goddamn tune is just flat out ridiculous if you ask me...

[Zod Wallop]

Depends on your phone and company. I have Cingular and my phone will accept any MP3 as a ringtone.
I just email myself the MP3 as an attachment. For Cingular, it's Yourphonenumber@mms.mycingular.com. Right now I'm using part of They Might Be Giant's 'Particle Man' as my ring tone.
Can't guarantee this will work for you though.