If you have a OpenWRT-capable router with USB and a USB Soundcard laying around, you can now use it as a AirTunes compatible reciever.

I have built packages for bcrm47xx and ar71xx type SoC's for OpenWRT Barrier Breaker

Atheros AR71xx (for example TP-Link WDR 3xxx/4xxx)
shairport_new_0.08_ar71xx.ipk

Broadcom 47xx (for example Netgear WNR3500L)
shairport_new_0.08_brcm47xx.ipk

Thanks to sm3rt for OpenWRT packaging: https://github.com/sm3rt/OpenWRT-ShairPort

If you are running Linux on your clients, you can always use PulseAudio for network audio streaming.

Update

Shairport and Shairport-sync is now included in OpenWRT 15.05. Just install the packages :)