There are a handful of Android apps I find useful, that are no longer (as of 2015-07-12) available from the Android market.
Fortunately, I can just download the .apk files and install them. (and stash them on my website)
apkshare
Given an already installed app, uploads its .apk somewhere.
Hilariously, this app itself is gone from the Android market but I had the foresight to use it to make a backup of itself.
com.kayan.nurav.apkshare-1.apk (112,872 bytes)
tricorder
Pretends to be a Star Trek tricorder, but is actually really useful because it shows detailed numbers for the accelerometer and magnetometer.
An earlier version of this app got in trouble for using the LCARS interface. This version doesn't have the rounded corners.
biz.lakin.android.apps.tricorder-1.apk (655,808 bytes)
audalyzer
Realtime voice print / audio spectrum analyzer. Lots of fun, but also genuinely useful for tracking down real-world objects that squeal at high frequencies like power supplies and fire alarm speakers.
org.hermit.audalyzer-1.apk (302,425 bytes)
swiftp
FTP server. I used to use this to transfer files off my phone in a sane way since MTP is a shit-show.
Virustotal thinks this APK has a virus, so I've double bagged it in a ZIP file. The password is "password"
If you're worried about this, consider SSHelper instead.
Otherwise:
org.swiftp-1.apk.password_is_password.zip (66,481 bytes)
Other resources
APKMirror has older versions of some apps.