What are some good replacement ICs for DIP UARTs?
Newark/Farnell have 1,931 units of NXP's SC16C550 available. Stock up now!

Other possibilities include the SC16C2552 and SC16C752. 

"But they aren't DIP!" you say. Well there is simple solution for that:-

SMT Breakout PCB for 48-QFN or 48-TQFP   



I would just use the SMD part directly though - saves board space and is easier to solder than DIP.