Minimum Tranasction Fees At current exchange rates, it looks like the standard transaction fee is on the order of tens of cents (USD).  Inclusion of the standard transaction fee means getting the transaction picked up within minutes.  

Conversely, zero transaction fee transactions may not be picked up for days.  It also seems to be the case, at least for now, that transactions in limbo like this may be double spent by reissuing with a higher transaction fee.  See earlier discussion here:  Could any standard fee transaction be double spent by using a higher fee?

One can imagine that in the longer term there might be a market equilibrium in which a certain floating minimum transaction fee is required by miners to guarantee processing, and that sellers might not deliver/perform unless that fee were included in a payment.

Are there ways to estimate what the minimum fee might be?
Bitcoin Core, as of 0.10, has a fee estimation RPC call, which you can read more about here: http://jonathanpatrick.me/blog/floating-fees .

If you don't want to run Bitcoin Core, there are other third party services that provide simple API calls to estimate fees as well, like the Chain Endpoint on BlockCypher's API: http://dev.blockcypher.com/#chain-endpoint (Full disclosure, I work there/am biased)