virtual piano keyboards can be useful if you're a singer trying to learn a part from a score (whether for choir, opera, musical theatre, pop or rock) but you don't have access to a musical instrument. All you need is a computer with soundcard and speakers or headphones / earphones.
Here are some examples of free online piano keyboards available through your web browser via the Flash technology or a Java applet (which all modern web browsers can handle); two can record and play back a series of notes, one of them can even be downloaded to your computer (whether desktop, laptop or notebook) and then played offline without any internet connection! They're meant for educational games, teaching piano rather than choral or vocal lines, but are in fact very handy for vocalists too.
(For those who don't play the piano, there are lots of diagrams on the Net of which keys on the keyboard correspond to which notes on the music staff, see e.g. this pic or this more comprehensive one)
1. AbabasoftVirtual keyboard and free downloadable Flash version (just rightclick that link to save it to your computer; in future, doubleclick the saved .swf file to access the keyboard).
You can click on several keys in a row (including rests if desired) and they'll show up in the stave below; the Play button then replays all of them - good for learning a sequence of notes, and Loop will repeat it over and over to really get the pitches into your head. Temp. is self-evident, you can set the speed so it plays back faster or slower. Sound's a bit too sustained for my taste, but that's just a quibble.
Ababasoft also have a "chord piano" (again downloadable) which lets you hear your part against other people's (in vertically stacked boxes) - click in a box, then click the keyboard to set the note for that box. Double left or right arrow buttons move you along horizontally one stack at a time, and what you hear is always what's in the middle stack at the time.
2. FlashmusicgamesNo music stave but the Flashmusicgames piano (again downloadable) lets you set the duration of the notes played, in milliseconds (default is 100 ms), if you prefer less sustain per note than Ababasoft's piano.
3. ApronusBare bones Flash pianoforte, no way to record and replay a sequence so it's "live" only, but it gives you 2 octaves and a more realistic piano sound.
4. PianoworldWai Man Wong's Java applet synthesiser keyboard (with piano sound), don't be surprised if it plays you a back a fast run while you're waiting for the page to come up!
2 octaves, hit Rec before you play it in order to record, Ply to play back what you recorded, Clr to clear the recording. (Not figured out Mem, it just plays a B - as does clicking anywhere on the synth picture above the keyboard other than Demo or Drum..)
Bonus fun bit: click "Drum" to hear a random drum sound; you can even intersperse your recording of notes with drum sounds, but sadly when you play back the recording you can't accompany yourself with the drum live!