Which bitcoin wallets let you export to a universal .wallet format? Which bitcoin wallet applications for android, windows, and mac allow you to export .wallet files that are cross-compatible with each other? Currently there are so many wallet applications out there and I just want to use as few as possible that give a sense of security if one day the app developer just stops developing for their app.
Thin Clients (SPV)


MultiBit.org


Tools -> Export Private Keys -> Open the file you exported

grabHive.com (Mac only)


Export not yet implemented



Browser Add-ons


SpareCoins.io (Chrome)


Wrench icon -> Backup Wallet (encrypted)



Full Clients


Bitcoin-Qt


Help -> Debug window -> Console


walletpassphrase {passphrase} {timeout}
dumpwallet {filename} (Recommended method, dumps all wallet keys in a human-readble format, into a file named filename)
dumpprivkey {bitcoinaddress} (Optional method, exports a private key)




Clients with Deterministic Wallets


BitcoinArmory.com


Wallet Properties -> Backup Individual Keys -> Private Key (Plain Base58)

Electrum.org


Export -> Private Keys -> Open the file you exported



Hybrid Web Wallets


StrongCoin.com


Your Accounts -> Select one -> Decode Private Key (with passphrase)

BlockChain.info


Import/Export -> Export Unencrypted -> Look for 5*, L* or K* Private Keys



Android Wallets


Mycelium.com


Keys -> (Options bar |) -> Export
(OUT OF DATE) Key Management -> (Options) -> Export to SD card(jpg to "mycelium-export" folder), QR code or to the clipboard

BitcoinSpinner


(Options) -> Settings -> Advanced -> Export private key -> QR code or clipboard

Bitcoin Wallet (by Andreas Schildbach)


(Options) -> Back up Keys (encrypted)


openssl enc -d -aes-256-cbc -a -in 





  .... there will be more in time, hope to keep this answer updated