Let's say a user adds some products to a cart and we'd like to distinguish the items that were just added compared to the items that were already in the cart.
Or a user uploaded some files into something like Google Drive and we'd like to visually distinguish the new files.
For example, in forums, it's very common to put the title of the thread in bold to indicate a new or unread post in that thread.
Are there any other conventions or research done about this?
As evidenced by the answers here, and in other UX.SE questions, there are many ways to go about it. This gives us a hint that there are no standards, conventions, or guidelines to follow -- though it doesn't preclude those forming in the future.
The absence of such standards is not surprising: a list can have many representative forms -- bulleted, with or without images and thumbnails, descriptions, attributes, and so on -- and has to adapt its specific form to a variety of different environments and needs.
Thus, the best we can do is turn to research on generic ways to differentiate items in a list. In 2016, Nielson Norman Group measured that:
Users are roughly 37% faster at finding items within a list on a web page when visual indicators vary both in color and icon compared to text alone. If choosing between using color or an icon, icons with strong information scent perform better than color alone.
From this research, we can take away that if the attribute of "newness" is the most important differentiator in your list, then use variations in color and icon to highlight the difference. Color alone helps, but does not perform as well as both.
You can see that the examples in other answers use this technique, suggesting that common implementations follow the research.
When faced with this problem, my recommendation would be to evaluate your list's specific form and introduce variation in color and icon to highlight the most important attribute. If there are many important attributes, provide a way to sort and filter.
Microsoft uses an icon to show when something is new (rays that show "shiny"). Here's an example taken from their website:
I'm not sure that there's a universally understood pattern for this, so testing with your particular audience might be good.
Notification feeds (e.g. Gmail, Reddit, etc.) often use a different background color to indicate unread. That also gives you the option to adjust the color based on factors such as why or when it was added.
No research but I'd prefer some sort of tag (e.g. NEW ITEM or NEW IN CART [Thx @wizzwizz4, for the hint!]) or clear text: New item in cart. I'm not sure whether bold text can be easily distinguished from normal one by visually impaired and on all conceivable devices.
And bold text can mean different things in different apps: selected for further action, highlighting a special offer, item with personal discount or, well, new item at this place... you name it.
Acoording to Visual Indicators to Differentiate Items in a List: "Both [icon and color] Is Best".
In Apple's Mail app, unread, therefore new messages are denoted with a blue circle next to the email subject line.
Mail applications (for example, the Apple Mail application) sometimes used a colored dot to indicate a new (or unread) message.
I've seen a many-pointed star used for this, e.g.: 🟓 (perhaps not rendered correctly due to your browser missing according font - click on the link to view it). A similar symbol is commonly used on retail packaging to attract attention to new features.
Unicode also offers🗦These🗧emphasis rays. Similar symbols are often used to highlight changes in comic strips and visual novels.
Or even a simple (or fancier) exclamation mark.
NEW ITEMtag mentioned in one of the answers.