When I was still running Windows I used Foobar2000. The mass tagging/transcoding tools were pretty nice, it had a decent graphical EQ, and it had a simple UI that didn't get in your way.
On Linux I use Audacious, which is a Winamp clone for Linux, more or less. Its EQ feature kind of sucks, but now that I've figured out how to install global EQ plugins into the Linux audio stack using JACK I don't care.
For ripping/encoding CDs I use the Linux Sound Juicer app to rip to FLAC, plus assorted shell scripts I wrote myself for transcoding to MP3/OGG via the LAME/Vorbis CLI tools. (My current portable player is a 4GB Sansa Clip+ with a 16GB micro-SDHC card in the external memory slot; 20GB of storage + 160kbit/sec OGG encoding means I can get a substantial chunk of my music collection onto the player with fidelity that is plenty good enough for listening to in the car.)
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