I suppose I was thinking of Java and .NET where you don't generally get the chance to "pick" a hash function unless you're in the Crypto namespaces. There are certainly hash-based data structures like HashTables and the like, but the underlying hash algorithms are not exposed in any way, they're just magic.

Not to pile on with the nitpicking, but for just the cases everybody here is talking about (eg non-crypto hashing) you get to "pick" exactly the hash function. You're forced, actually, to implement hashCode() in Java if you want to be able to put the object in a HashMap, HashSet, etc.

You're right but that's still in a very small subset of possible hashing functions because you must produce a 4 bytes hash, which is then hashed again (to the length of the storage array). Of course you don't have as requirement to avoid collision, just to distribute as equally as possible.

You can override the default hash producing methods in both Java (hashCode) and C# (GetHashCode) - so while the standard hash generators might not be exposed directly they can easily be replaced.

Using chosen hash functions in your applications isn't so rare even outside cryptography field. For example I often use hash for storage addressing (ala git). For this kind of thing you need something fast and with practically no collision (which isn't a requirement for a common hash table).