Over the weekend, Niantic Labs quietly released a server side patch to Pokemon Go, making several tweaks in what many perceived as a "balance update". The patch didn't contain any major fixes to longstanding issues like tracking, but many players were pleased by the perceived updates in gameplay.
The biggest change to Pokemon Go was the sudden influx of uncommon and rare spawns in all areas. Most impacted by the change were rural and suburban areas, which traditionally have had significantly less Pokemon than densely populated urban areas. Players also noticed that "region locked" Pokemon like Kangaskhan and Mr. Mime were also appearing with greater frequency on their respective continents.
Niantic also changed the movesets to several Pokemon in Pokemon Go, leading to some minor adjustments. Notably, Gyrados lost the ability to use Dragon Breath, Chansey and Jigglypuff both gained more effective charge attacks, Koffing and Weezing lost the ability to use Acid, and several Pokemon's moves were switched with other attacks. /r/TheSilphRoad, a Pokemon Go community on Reddit, compiled all the minor changes into a nifty infographic that can be found here.
There also appears to be a minor tweak as to how Pokemon Go tracks distance while hatching eggs. Traditionally, Pokemon Go determines a player's speed and distance traveled by periodically pinging a player's location and comparing it to their last location, which can result in the game miscalculating distances if the player's not traveling in a straight line. The game also stops tracking distances when the player travels faster than 10.5 kilometers per hours, in order to stop players from driving around with Pokemon Go open in order to hatch eggs faster. Players noticed that Pokemon Go started tracking distance much more accurately over the weekend, presumably due to more frequent location checks. Some sites have also prematurely reported that the game's egg hatching "speed limit" had also increased, but those reports seem premature at best.
Niantic has previously pushed server side balance patches shortly before a new update, so it's likely we'll see a new version of Pokemon Go hit app stores sometime in the next few days.