What are some infill interchanges you know of?

[Bruce]

Off the top of my head, for the Seattle area:

I-5
HOV-only exit at 112th Street in Everett - opened in 2011 to serve a new park-and-ride built in the freeway median
Exit 200 (88th Street, Marysville) - opened in 1997 to relieve congestion at SR 528 and spur development of Quil Ceda Village

I-405
Exit 13A (4th Street, Bellevue) - opened in the 1980s

I-90
Exit 18 (Highlands Drive, Issaquah) - opened in the mid-2000s to serve the new Issaquah Highlands neighborhood; an existing ramp was there but went towards downtown

SR 167
24th Street E in northern Sumner - opened in 2004

SR 520
40th Street in Overlake - opened in 2000ish to serve the Microsoft HQ

[jakeroot]

Couple more in the Seattle (more Tacoma) area:

WA-512
* Half-interchange at 9th St SW (94th Ave East) in Puyallup was added in the late 1990s. The full interchange just next to it (at WA-161) was then modified to remove a couple movements  (Google Maps).

I-5
* The Center Drive interchange in Dupont was added around 2000 (Google Maps), and was then modified in 2014 to better incorporate a JBLM access road.
* Tumwater Blvd interchange was added in the 1980s (Google Maps).