A web page shows that many crypto currencies lack protection against 51% attacks. For example to double spend in Dash, one needs to pay only 14K$ per hour. To double spend in bitcoin private, 1000$ will suffice. And course benefits of double spending can easily outweigh the costs.
In addition some of the higher numbers are questionable. There is almost certainly an easier way to command 51% of bitcoin hash rate for one hour than paying 650K$. It is sufficient to hack some pool servers, or directly the software run by miners. Luckily for bitcoin, there exist vaste privately-owned mining farms where the software and the hardware are also proprietary.
ADDED: This is how Bitcoin Gold has lost all credibility.
ADDED LATER: And this is how ETC has lost credibility also.
ADDED in 2020. New stratum protocol will have some protection against hash redirection attacks.
ADDED in 2020: Some recent research on 51% attacks on POW and POS blockchains.