A FLAC library is great right up until a player refuses it. FLAC to MP3 starts when you drop files anywhere on this page: each one is decoded, encoded with LAME at 320 kbps by default, and saved as an MP3. Batch your whole collection; nothing uploads.
The honest quality note
FLAC is lossless; MP3 isn't. The default is 320 kbps, the highest MP3 allows, so you never have to wonder whether you gave up quality. 256 is still inaudibly different for almost everyone on almost any gear, and 192 sounds great if you want smaller files. Keep your FLACs as the archive; carry the MP3s.
FAQ
Album tags and artwork?
Version one carries the audio only. Tag preservation is on the workbench. Your FLAC originals are untouched either way.
How fast is a big batch?
Roughly faster than the upload alone would be elsewhere. Encoding runs at several× realtime on a typical laptop.