This is a thorny topic and really depends on what you mean by "usable" and "lost to accidents".  Of the oft-cited 21 million BTC total:

• some are fully spendable today.
• some have yet to be generated (do not yet exist).
• some are "lost" because a corresponding private key was lost (8999 BTC example by Stone Man) but are spendable in principle (cryptography breakthrough, quantum computers).  Some of these will be easier to find than others: Casascius coins for example made us of the mini private key format and so are protected by 128 bits of entropy rather than the usual 160 bits.
  
• some are at addresses for which it is not known that a corresponding private key exists (1111111111111111111114oLvT2; The Bitcoin Eater address; cryptograffiti.info/)
  
• some are unspendable due to protocol bugs (the 50 BTC in question and the 100 BTC mentioned by zebedee above)
• some are locked by scripts which provably return false on any input.  Some of these are so badly lost they don't even have addresses! (367.75849319 BTC example).
  
• 23.1 mills (23.1 millibitcoins = 0.0231 BTC) may well never be mined into existence in the first place due to rounding all subsidies down to the nearest satoshi.
• some have already lost the chance to ever be mined into existence (example by midnightmagic: block #124724 mints only 49.999 999 99 BTC).
  

Note: I use "exist" here for any amounts of BTC in an up-to-date unpruned Bitcoin UTXO database.  There are of course different interpretations of what it means for a bitcoin to exist.  Even this seemingly simple definition is not water-tight as two different full nodes may disagree on the UTXO and in particular it allows one to claim that at one point, there existed more than 180 billion bitcoins.