I'll start, and others may comment or chime in.
1. ReplayGain. This tells you how much you need to amplify your recording to get it near the 89 dB target volume. Instructions here: https://wiki.librivox.org/index.php?title=Measuring_Volume_within_Audacity
I do find its results get me a little low, so I overshoot it a little bit. If it tells me I need to amplify by 0.8 dB, I'll definitely do 1.0 or even 1.5 dB.
2. DeClicker. This will almost miraculously remove the little annoying clicks in the recording, either in the silences or even within words themselves. Forum thread about it is here: viewtopic.php?f=23&t=52753
It's powerful, though, and can remove some T, K, and D sounds, so it must be used sparingly. I use it as I'm editing, to highlight a small portion of recording and remove clicks from that, rather than over the entire file.
3. Limiter. This is already built into Audacity. While the best way to smooth out your vocals (not blast out certain vowel sounds, etc.) is to simply school your reading voice to be more even, this tool is handy for the times when you have spikes that you'd like to dampen down. It's also good to use when you need to amplify, but a spike (or a few) are too high and prevent you from amplifying enough.
I used to use Compressor for this, but it has a weird fade-in effect at the beginning of the track. I've found that Limiter does the same thing without the fade-in.
I use "Soft limit" and have "Limit to" set to -4.0 dB. The rest of the settings are the default. You may need to experiment to determine which setting is right for you. A basic rule of thumb is "lesser is better."

4. Noise Reduction. This one is pretty well-known. You tell it what is "background noise", then have it filter out that background noise to make a cleaner sounding file. It only works on constant noises (white noise). It doesn't clean out door slams, telephone rings, dog barks, etc.

These are the four effects I use on every recording. (Well, except for ReplayGain, because I know my volume pretty well by now.)
Anyone else have any must-haves?