Hi, I decided to do a short write-up on the history of United's crest for those who might not have read up on it yet, or those who find this sort of thing interesting...

Newton Heath

Manchester United was formed in 1878 as "Newton Heath Football Club" by the Carriage & Wagon department of the Railway depot at Newton Heath, Manchester. During this period the team consisted mainly out of workers from the railway company and competed against other departments, before joining a local league in 1888.

Manchester United

In 1902 the club was £2,670 in debt and was served with a winding-up order. Captain Harry Stafford and found four local businessmen each invested £500 in the club, saving it from closing down. As a mark of this fresh start the club's name was changed to "Manchester United".

1st Crest

In the early years of football, club crests were not worn on the shirts of players. Crests were only embroidered onto the shirt to mark special occasions like cup finals. Manchester United wore a Lancashire red rose on the shirt during the club's first major cup final, which was the FA Cup final against Bristol City in 1909, which the team won 1-0.

2nd Crest

Manchester United's second crest was based on the Manchester City Council coat of arms, and made its first appearance in 1948, when United played in their second ever FA Cup final (against Blackpool) which was won 4-2. This is how the crest appeared on the jersey for that game. The same crest was also worn like this for the 1957 Cup final against Aston Villa.

3rd Crest

In the aftermath of the Munich air disaster of 1958, United managed to reach the FA Cup final again and wore this crest on their jerseys against Bolton Wanderers. Contrary to popular belief, it does not depict a phoenix rising from the ashes, but an eagle and was based on a newly introduced badge for the City Council. You can see this eagle in Manchester City's badge today.

4th Crest

When United reached the 1963 FA Cup final (playing Leicester City), the club reverted back to the Manchester City Council coat of arms, albeit a newer, more updated version. It was also used for the 1968 European Cup final.

5th Crest

In the 1960's this new version of the crest made its appearance in official club documents. The flower, ship and stripe elements were inspired by the original City Council coat of arms worn previously. The three bands are said to represent the city's three rivers, the Medlock, the Irwell and the Irk, although it is more probable that they represent the ancient arms of the Grelley family (the first Barons of Manchester). Above that is a ship in full sail, a symbol of trade and enterprise. This version of the crest features roses, presumably representative of the red roses of Lancashire (which Manchester was still a part of at the time), but for some reason they have not been colored in and could therefore be confused with Yorkshire's white roses.

6th Crest

In 1971 a redesigned version of the crest appeared for the first time as a permanent badge on the shirt of players. This is the first version of the crest that features the red devil, referencing the nickname that the club has garnered by this time. Here and here are some more examples of the crest during this period.

7th Crest

In 1980 Adidas replaced Admiral as the kit supplier, which led to a similar, but slightly updated version of the crest being used. Adidas added a pair of three-stripe boots to the top ribbon of the crest, an addition that remained until 1992.

8th Crest

In 1992, the inception of the Premiere league coincides with Umbro taking over from Adidas as the kit sponsor. The crest was updated again, with the boots being removed along with the solid white background.

9th Crest

In 1998 the most controversial change to the crest was made. The words "football club" were removed and the colors were altered. The badge was also squashed horizontally and a black outline was added. This is how the new badge looked. This is the badge we know today. Some more examples.

^{Sources: Wikipedia, Soccermaniak.com, Thebeautifulhistory blog, RedCafe.net}