There are a couple of “Music Utility” websites I keep in my bookmarks bar if I need to quickly make use of a web browser as a practice aid.

BPM Tool for quickly semi-manually figuring out the tempo of anything you’re listening to – I use the super simple http://www.tempotap.com/ – Press X to “start/reset” and then press the space bar to tap each beat. It even sort-of works on mobile devices..

Drone Tool – This site plays a single chord as a kind of a drone. It is called “The Chord Sounder™”. Pretty neat / simple idea and tool. Of course, if you want to actually practice “making the changes” this is not the tool for that job.

Online Metronome – Depending on what I feel like listening to, I’ll either use the Metronome Online site or this online metronome from 8notes.com.

Online Tuner – There are a couple of online tuners, but these get the extreme caveats of “Your Mileage May Vary” – I’ve had OK luck with this online tuner from Jamplay as well as this other one.

Non-Musical Bonus Mention of a web app I’m always grabbing for –  http://copypastecharacter.com/ Doesn’t work always, but often it does.