How to uncompress zlib data in UNIX?

It is also possible to decompress it using standard shell-script + gzip, if you don't have, or want to use openssl or other tools.
The trick is to prepend the gzip magic number and compress method to the actual data from zlib.compress:

printf "\x1f\x8b\x08\x00\x00\x00\x00\x00" |cat - /tmp/data |gzip -dc >/tmp/out

Edits:
@d0sboots commented: For RAW Deflate data, you need to add 2 more null bytes:
→ "\x1f\x8b\x08\x00\x00\x00\x00\x00\x00\x00"

This Q on SO gives more information about this approach. An answer there suggests that there is also an 8 byte footer.

Users @Vitali-Kushner and @mark-bessey reported success even with truncated files, so a gzip footer does not seem strictly required.

@tobias-kienzler suggested this function for the bashrc:
zlipd() (printf "\x1f\x8b\x08\x00\x00\x00\x00\x00" |cat - $@ |gzip -dc)

zlib-flate -uncompress < IN_FILE > OUT_FILE

I tried this and it worked for me.

zlib-flate can be found in package qpdf (in Debian Squeeze and Fedora 23, according to comments in other answers)

(Thanks to user @tino who provided this as a comment below the OpenSSL answer. Made into propper answer for easy access.)

I have found a solution (one of the possible ones), it's using openssl:

$ openssl zlib -d < /tmp/data

or

$ openssl zlib -d -in /tmp/data

*NOTE: zlib functionality is apparently available in recent openssl versions >=1.0.0 (OpenSSL has to be configured/built with zlib or zlib-dynamic option, the latter is default)