UPDATED SPAWN LOCATIONS!!!! This post is awesome because it shows what spawn locations have turned into. I can personally verify several of the spawns below, it seems like anywhere you used to see a specific Pokemon at, they are now switched to spawn whatever else is on the table in the link above.
Shellder (maybe Ghastly also?): Amundson Park, off of Elk Grove Florin Road between Calvine and Sheldon. He may only spawn once every hour or two, fifteen minutes past the hour. Someone caught him at 3:15pm recently, and I caught mine at 1:15 several weeks ago. I've heard there are 3-4 Shellder popping up at a time!
Krabby/Voltorb - I think Voltorb in this case: Shasta Community Park, across from CRC. Personally I found mine at the Northwest corner of the park near the library. Not sure if there is more than one that spawns around here, but there is at least one!
Magnemite/Doduo: Backer Park, way down South between Franklin and Bruceville Blvds on Bilby Road. There seems to be constantly one Slowpoke chillin here, usually by the parking lot or the Southwest corner of the park. I've gotten maybe 5-8 of these guys here so far, and am currently working towards my Slowbro.
Seel!: Wackford Sports Complex, off of Bruceville and Big Horn Blvds. This would probably be considered a "nest" as you can typically get 2-4 of these guys up at one time. They may stop spawning after dark, however. ALSO spawning at Hal Bartholemew Sports Park!
Paras...: Stonelake Clubhouse Parking Lot, off of Elk Grove Blvd close by I-5. I always see at least 2-3 of these guys hanging out in front when I drive by/through the parking area. I was able to get my Vileplume thanks mainly to this spot. BUT WHO CARES?!
Omanyte/Eevee: Elk Grove Regional Park, off of Elk Grove Florin Road by 99. I haven't been out here myself but I've always seen those lil fish flopping around this park since the start of the game. Probably at least 1-3 always up at one time, around the the whole park. They may actually not have that high of a spawn rate!
Jynx: Sam's Club parking lot, off of Calvine Road and 99. Not the most exciting Pokemon, but there are plenty of these guys to be found all around the parking lot.
Abra!: Land Park, specifically around Fairytale Town. It's off of Sutterville Road and Land Park Drive. You can grab several by either driving around this area and the parking lot behind Fairytale Town, or just walking between the zoo and Fairytale Town.
Electrabuzz/Jynx: Freemont Park in Downtown between 15th-16th streets and P-Q streets. You can generally get one or two in the middle of the park.
Magmar/Electrabuzz: Maidu Park out in Roseville. I haven't been out here personally but there seem to be ample Pokemon around!
Machop: West Sac neighborhood kinda behind the Target off of Jefferson Blvd. Shoutout to /u/Chunkyschmonkey from this post here. Updated so that Machop is spawning here now!
Lapras: Independence Park - Vallejo, located somewhere on the water, but he appears to be a rare spawn that barely ever shows up. I know this is way outside of Sac, but it's a fucking Lapras! Thanks to /u/RainManRaiding and this post.
Magikarp: Natomas Regional Park, off of Del Paso Road and Natomas Blvd. Several of them have been popping up around the path. Thanks again to /u/RainManRaiding from this post. Updated to show Magikarp here.
Bulbasaur/Charmander: Tahoe Park, off of 65th street between Broadway and 14th Ave. I have read you can consistently catch at least one of these guys here on a regular basis. With the nest update of 8/22, supposedly now both Charmander and Bulbasaur pop up here.
Staryu: West Sac River Walk, just on the West side of the Tower Bridge. I always see at least a couple of these when I walk from one end to the other.
I think that's all I got for now, feel free to add some in the comments and I'll get em added up. Also as always check out this map, I believe it is rather up to dateIT'S OUTDATED! Use the link at the top to see the table of what nests turned into what: https://www.google.com/maps/d/viewer?mid=15VjXgE66QW1vZuUFJBpNkliOuiE&hl=en_US