I came here to suggest LeetKey too. It's client side, and supports a number of reversible transformations. Most of the encryptions work with all types of punctuation (morse/rot13 being the exceptions). Some people might be able to decrypt rot13, a handful morse, and a couple hexadecimal; nobody is going to accidentally read AES encryption. It supports password based encryption/decryption for those encryptions that require it. Also, it's free and easy to use. The following encryptions all took about 2 seconds each. If we were all using leetkey, swapping to a new spoiler type would be as easy as picking and choosing.

Here's a spoiler in Rot13. Urer'f n fcbvyre va Ebg13.

Here's a spoiler in base64. SGVyZSdzIGEgc3BvaWxlciBpbiBiYXNlNjQu

Here's a spoiler in AES encryption. (Key: lesswrong)

XqqfeDMBITAhITAhLV5g2FnewrRa20y8iuzAA/AttnBO8f40jPAUYDulZnYyVi16mco=

You can also type in reverse, if you really want. .tnaw yllaer uoy fi ,esrever ni epyt osla nac uoY

Edit: Removed binary, morse, DES encryption, etc because they looked really annoying on the page and that many examples were unnecessary.