Who and Who Did Ashwamedh Yagya?
There have been many kings whose Ashwamedh Yagya are famous in Indian mythology, but the following Yagya are more famous than others as they have something other incidents connected to them

Sagar
Raajaa Sagar was the --- of Raam. He was in the family line of Harishchandra - Harishchandra > Rohit >> Baahuk, and Baahuk's son was Sagar.  He also did an Ashwamedh Yagya in which Indra stole his Yagya horse  Its description is given in Bhaagvat Puraan, 9/4. Sagar had two wives, one of them had 60,000 sons, and the other one had only one son Asmanjas. Asmanjas had one son Anshumaan who was very dear to Sagar. First Sagar sent his 60,000 sons to find his horse. Incidentally they found it tied near Kapil Jee. They thought that Kapil Jee had stolen it so they abused him and he burnt all of them just by looking at them. Then he asked Anshumaan to find his horse and his 60,000 sons. Anshumaan found both - his uncles in ash form and horse in Kapil Muni's Ashram. he brought the horse back and then only Sagar could complete his Ashwamedh Yagya. To give Sagar's 60,000 sons good Gati, His great grandson Bhageerath brought Gangaa on Prithvi.

Raam
Raam's Ashwamedh Yagya is very famous in which He did not have His wife and He had to keep eetaa's golden statue beside Him to complete the Yagya. And during which He met His sons, Kush and Lav, also. Later He called Seetaa also to join Him and then He again put Her to test and as Seetaa was tired of these tests, She went to Her place from where She came. A very detailed description of this Yagya is given in Padm Puraan, Paataal Khand.

Yudhishthir
Yudhishthir did his Ashwamedh Yagya after the war and becoming the King of Hatinaapur. He collected the money for it from the place where the Braahman left their Dakshinaa material got from Raajaa Marutt. Its description is give in MBH, Ashwamedh Parv.

Who Performed 100 Ashwamedh Yagya
There are many kings who have performed 100 Ashwamedh yagya. It is said that who has performed 100 Ashwamedh yagya, he could be appointed as Indra.

(1) Maandhaataa
Son of Yuvanaashwa of Soorya Vansh, Maandhaataa performed 100 Ashwamedh Yagya

(2) Yayaati
Raajaa Yayaati, Nahush's son, performed 100 Ashwamedh Yagya - that is why Indra was afraid of him and tried to fell him from his good path.

(3) Nahush
Nahush also did 100 Ashwamedh yagya and became Indra. What he did after becoming Indra. is known to everybody.

(4) Daitya Raaj Bali
He was also about to complete his 100 Ashwamedh yagya that Vishnu had to stop his 100th Yagya as if he had completed it he would be the king of Swarg and Vishnu had to give Swarg to Indra at that time. So He assumed the Vaaman Avataar, stopped his 100th Ashwamedh Yagya and sent him to Sutal Lok till he would be next Indra in next Manvantar.