Below is a listing of 25mm/28mm miniatures for post-WWII and Modern conflicts. I have avoided those ranges which are "near future" ranges intended for the sci-fi/zombie market, and focused on ranges of interest to historical miniatures wargamers.
I have no commercial interest in any of the lines listed (other than having spent more money than I should on some of them!) The listings here are not reviews or recommendations - wargamers will always make their own choices based on their preferences/needs. (The good news is that we have a lot of very nice figures out there to choose from.)
This list is as complete as I can make it - please contact me if there are any ranges missing. Order is alphabetical. This list is as of December 2018.
The Assault Group: Producers of a 28mm Ultra-Modern range covering many nationalities and a Vietnam range ("Up Country").
Baker Company: Baker Company has a range of 28mm Vietnam figures, including ANZACs and civilians, and some vehicles (MUTT, M-113).
Battle Honours: A 25mm Vietnam line produced by 19th Century Miniatures in the US (some of the Battle Honours line is sometimes sold as Rank & File in the UK by Timecast). These are slighter figures than some other more modern offerings.
Britannia Miniatures (Grubby Tanks): A variety of 28mm ranges including the Malayan Emergency, Mogadishu, the Congo, and S.W.A.T. figures.
Brooks Miniatures: A 28mm line available from Old Glory UK covering the British in Afghanistan (including insurgents and Afghan government troops).
Dishdash Games (Skirmish Sangin): 28mm figures for insurgents, US, and ANZAC forces. Publishers of Skirmish Sangin and other similar rules.
Empress Miniatures: Not only does this company produce a wide range of 28mm figures in their "Modern Combat" line, including French, Bundeswehr, Russians, Chechens, Chinese, and more, but they also have a line of figures for the French Indo-China conflict in their "Vietnam" range.
Eureka Miniatures: This Australian company is known for producing figures which no one else makes. Their Moderns line includes figures from Aden and Yemen along with Germans, Soviets, Australians, and other more usual figures. Under their "Kreigspiel" brand they also have Australians, US, and Musorians (non-specific Soviet-equipped troops). These figures are available from Eureka Miniatures USA in the US and Fighting 15s in the UK.
1st Corps: These guys do a very small range of 28mm figures for the Korean War, including Chinese and British squads.
Full Battle Rattle Miniatures: This company produces ISAF Canadians (2006 and on) in 28mm.
Irregular Miniatures: Irregular make 28mm insurgents in their "Moderns" line.
Jerwood Miniatures: Jerwood has 28mm modern Estonians, Baltic special forces, and the old Venexia French and Italians.
Militia Miniatures: A line of 28mm mercenary character figures.
MoFo: This line of 28mm moderns, available from Badger Games LLC in the US, includes forces for the Falklands War, African militia, US special forces, and mercenaries. Available in the UK from Gripping Beast.
Mongrel Miniatures: Mongrel Miniatures produce figures primarily (but not exclusively) for 1980s conflicts, including East Germans, Soviets, Israelis, British, US, and some less-common figures such as modern Pakistanis. They are available from Badger Games LLC in the US (I have had great service from these guys).
S & S Models: S & S have a 28mm L118 105mm artillery piece along with some different crew sets in a variety of headgear.
Special Artizan Service Miniatures (SASM): An impressive range of 28mm (almost 30mm) miniatures, covering many recent conflicts: Ukraine, ISIS, etc. Some vehicles.
Spectre Miniatures: A broad range of 28mm miniatures with a focus on special forces and covert ops. Has a line of useful vehicles and some unusual "scenario" figures.
Stan Johansen: The 28mm Jihad line features US, IDF, Afghans, insurgents, and more. (Not to be confused with the 20mm line also listed on the site under the same name.)
Tiny Terrain Models: 28mm Russians and Chechens in their Chechen Wars line, and some other unusual figures in the Modern Warfare line (I like the Raven Drone set).
Under Fire Miniatures: Features Cold War 1980s East and West Germans, US, and Rhodesian SAS figures in 28mm.
Victory Force Miniatures: 28mm US Army, Marines, and insurgents in the Wars on Terror line.
West Wind: Listed as 28mm but really more like 25s, West Wind produces a broad range of Vietnam figures in their Nam Figures range, including civilians and some unusual figures. (Unlike other West Wind lines, this is no longer available from Old Glory in the US).
White Dragon Miniatures: The Courage In Contact line has 28mm modern British cast in polyurethane resin.