To relieve myself of the ambient noise at my work desk, I've started wearing closed-back isolation headphones. Since I need to listen to at least two different audio sources at the same time, a decent desktop audio mixer was in order. The last time I looked at mixers several years ago, there was nothing decent available at a budget price, let alone under $100. Today, there are several choices, and in my opinion, none better than the Behringer UB502 .
The Behringer UB502 is a 5-channel mono or 2-channel stereo mixer with a bidirectional tape loop. It has a powered microphone input, and most importantly for my desktop, separate mix levels for headphones and speakers. The sound quality is pristinely transparent, and the build quality is totally solid. Unbelievably, this mixer is only $35! And if you need more channels, Behringer makes other models with the same high quality.
Correction: There is a single master Mix volume and an additional Headphone volume. If you have the mix output sent to live speakers, the mix volume will go to both the speakers and headphones, but only the headphone volume is adjusted separately. This is a normal mixing board configuration. All this means is that if you are using this with desktop speakers, you'll want to keep the speakers' power on/off button handy when using headphones.