Depends on the city/county. Most in the US are on P25, which has been listenable by clued members of the populace, but many (most?) are now encrypting traffic (usually w/ AES, iirc).
Unencrypted transmissions are easily received by scanners (note, though, that older scanners can't pick up P25) -- they aren't cheap, though (I think mine was ~$450 USD). In my area, one local police department is the only agency that has chosen to encrypt their communications.
It is, supposedly, also possible to pick up P25 using RTL-SDRs, though I've never tried so I can't say for certain.
